Normal-incidence, high-speed, optical modulators are attractive devices for optical fiber communication because they can be interfaced to optical fibers more easily than waveguide-based devices. They are also interesting for integration with high-complexity Si CMOS because of their ability to provide large amounts of optical I/O directly to VLSI chips. There are various technological hurdles to be overcome before such devices would be interesting for widespread use in lightwave communications applications. The first of these is the production of modulator materials of sufficient quality to permit operation at the 1.55 micron wavelengths used in most modern telecommunication systems. A second obstacle is to design a modulator that will function with a modest applied voltage while still producing the high-contrast levels (10:1) desired in lightwave systems. Additionally, a significant temperature dependence of the modulation performance is observed when other operating variables (voltage, wavelength, etc.) are fixed. Furthermore, with the emergence of wavelength division multiplexed (WDM) systems, it may be desirable to operate normal-incidence optical modulators with sources that span a range of wavelengths, either near 850 nm, 1300 nm, or 1550 nm.
Normal-incidence optical modulators have an optical transmission (or reflection, for two-passes through the material) that depends upon the voltage applied to the device when the incident light falls within the proper wavelength range, as well as upon the temperature of the device, as a result of the temperature dependence of the semiconductor energy gap. For example, a reflective optical modulator used at room temperature might have a reflectivity versus wavelength characteristic similar to that shown in FIG. 4, and further described below, where the different traces correspond to different voltages applied to the device. It is desirable to obtain a large change in optical transmission from the modulator. Furthermore, in many cases one wishes to operate between two discrete states, one of low reflectivity and the other of high reflectivity. As can be seen from FIG. 4, different bias levels are more appropriate for different wavelengths of illumination. For instance, if we consider the change in reflectivity that can be obtained for operation at 855 nm, we might discover that the optimum bias levels to provide the maximum change in reflectivity are 0 V and 6 V. On the other hand, were we to operate at 860 nm, the optimum operating voltages would be different. Additionally, since the semiconductor bandgap shifts at approximately 0.3 nm per degree Celsius, the optimum operating voltage for a given wavelength will change with temperature.